How do I reconcile 97.6% from Medutest with 98.2% from the supplier?

Stated plainly: 97.6% · Medutest · 98.2%.

I noticed this today and I have not touched anything since, in case the state is diagnostic.

I have not discarded anything yet, so a test is still possible if that is the recommendation.

Should I be treating this as a failure or as noise?

0.6 percentage points, and the direction is the informative half. Medutest reports 97.6 per cent and the certificate 98.2, so the independent figure is lower. As impurity that is 2.4 per cent against 1.8 — 1.33 times as much unassigned area. A gentler gradient resolves impurities that a steeper one hides beneath the main peak, so the better method routinely reports the worse number; 0.6 points is comfortably inside what method choice alone produces on identical material. Ask both parties for the method section before you decide which figure is wrong, because the answer is often neither.

The single most important distinction is between what purity measures — the fraction of detected material that is your target — and what you actually want to know — how much of the material in the vial is your target.

Column pore size affects mass transfer — a 100 Angstrom packing on a 5 kDa peptide restricts diffusion, broadening the peak and potentially hiding small impurities in the shoulders.

Reconciling gross mass to label claim

Retention time is sequence-specific and method-specific, so comparing your result to a supplier value using a different method is meaningless without method documentation.

Compare purity within a single laboratory on the same method, never across laboratories.

edited 20 Aug 2025 by Dr_Hanne_Solberg — expanded the table to cover the lower concentration

The honest answer is that the achievable range of plausible purity figures for a given vial is wider than most people expect.

Temperature affects the dynamics of molecular conformation, and if a peptide has proline residues that interconvert on the chromatographic timescale, the peak will split or shoulder at low temperature and collapse at high temperature.

In practice, tailing factor measures peak shape, and a badly tailing peak spreads into the region where small impurities live, forcing tangent-skim integration that assigns tail area to the main peak.

The ICH Q3A impurity thresholds and the relevant pharmacopoeial chapters all specify method validation requirements that almost no research-grade certificate claims to meet.

I would be careful about over-reading a single measurement — treat it as a data point, not as ground truth.

The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.

The short answer is that two competent laboratories on identical material will disagree, and the disagreement is almost always explainable by method differences.

Mobile phase additive choice affects ionisation and peak shape — TFA gives sharp peaks but suppresses mass spectrometry signal, formic acid gives worse peaks but preserves signal.

Gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.

The caveat is that purity without identity is only half an answer — a high purity does not mean the peak is actually what you think it is.

If you are ranking vendors, specify a method and have all samples tested at the same place.

Concretely, the method matters more than the vial, which is why specifying a method buys you far more than changing suppliers does.

The fraction of your main peak that is actually your target versus isomers, fragments or related sequences is invisible without complementary identity testing.

The Arrhenius relationship for peptide degradation is the basis of accelerated stability testing and also governs how quickly methods drift with temperature.

One qualification: achieving purity above roughly 98 per cent on a 30-residue peptide is fighting the chemistry of synthesis, not the quality of the purification.

If you only pay for one test, pay for quantified content. Purity is the number everyone quotes and content is the number that changes what you do.

Stated carefully, identity and purity are orthogonal, and a high purity says almost nothing about whether the peak is actually what you think it is.

Buffer versus acid in the mobile phase changes the ionisation state of basic and acidic residues, shifting retention and selectivity — same vial, potentially different separation.

Worth noting that method standardisation is poor in the research-grade space compared to pharmaceutical work, so identical-looking methods can produce different results.

In practice: ask for the chromatogram, check the method section, check the lot number against the vial, and set your accept threshold before you see the result rather than after.